The present invention relates to the servo system art and, in particular, to a method and apparatus for automatically controlling the position of a device against a stop position with a controlled, limited force. The invention finds particular application in aircraft automatic throttle control systems.
An excellent example of a servoamplifier control device which must be driven against a stop, but with a controlled, limited force is found in the aircraft automatic throttle control art. In modern commercial aircraft, forward throttle travel is constrained to the jet engine main engine control forward stop. The angular range of the throttle lever is approximately 60.degree.. Normally, the throttle commands maximum takeoff or go-around thrust from the jet engine with eight to ten degrees margin to the forward stop, or at about 50.degree. to 52.degree. of angular throttle travel. In certain aircraft, such as Boeing model 737-300 and 737-500, jet engines exhibiting "fire-walled" thrust capability in excess of the nominal loading of the aircraft control surfaces have had their forward stop reduced by 8.degree. to 10.degree.. This keeps the thrust within the capability of the aircraft control surfaces. In the worst case tolerance scenario, the throttle is driven against the reduced forward stop during takeoff power.
An automatic throttle system drives the engine throttle through a clutch arrangement. The clutch allows the pilot to override the automatic throttle with a force no greater than that required to manually move the throttles. In the event that the clutch jams or fails to release, a secondary breakout detent mechanism is provided, whereby the pilot, at the expense of increased throttle loads, can override the jammed clutch by breaking out the detent mechanism to thereby control the engine throttle position.
The automatic throttle system experiences a problem when tolerances dictate that the required throttle position for takeoff is at, or slightly above, the forward stop at the main engine control. The automatic throttle, driving through the clutch and detent mechanism, tends to drive the throttle lever into the forward stop. At the forward stop, the autothrottle will increase the servo torque until the override detent mechanism is forced out of detent. Since the detents are several degrees apart, it may take several seconds for the automatic throttle to move the throttle lever position to the next detent in order to regain full control at which time the throttle will snap back a couple degrees.